Infrared detectors have use in a wide variety of scientific, medical, industrial astronomical, and military applications. In recent years, the field of IR photoreceptors has been advanced with the development of impurity band conduction (IBC) devices. Various implementations of these devices are described in "Blocked Impurity Band (BIB) Detectors, " U.S. Pat. No. 4,568,960 issued to Petroff et al.; "Solid State Photomultiplier," U.S. Pat. No. 4,586,068 issued to Petroff et al.; "Short Wavelength Impurity Band Conduction Detectors," U.S. Pat. No. 4,896,202 issued to Bharat et al; and "Intrinsic Impurity Band Conduction Detectors," U.S. Pat. No. 4,962,302 issued to Stapelbroek et al. The teachings of these prior patents are incorporated herein by reference.
The quantum efficiency of back-illuminated arsenic-doped silicon IBC detectors in the near IR region (wavelengths of about 1 to 2 .mu.m) is approximately 3% under optimal operating conditions. This low efficiency results from the small cross-section of the arsenic impurities in the active region of the detector at these wavelengths. A known approach for improving the quantum efficiency in the near IR region has been to illuminate the IBC detector on its edge. In edge-illuminated devices, the input radiation traverses a greater distance in the IR active region of the detector, thus providing greater opportunity for photon absorption by impurities in the active region, and thereby improving efficiency. In actual practice, however, the efficiency of edge-illuminated devices is less than the theoretically achievable because of the difficulty in focusing and aligning all the incoming radiation into the thin IR active region. Similar difficulties are encountered in coupling optical fibers to the detector because of the small dimensions involved. These difficulties are significant because they decrease the sensitivity of detectors that are required to detect very low levels of IR radiation. Thus, there is a continuing need to increase the quantum efficiency of detectors in the near IR region.